In systems for communicating high bit rate signals over a transmission line, a transmission amplifier or driver can be provided with a particular frequency-based gain. For example, when plotted on a frequency versus gain graph the gain can exhibit a particular shape. This is generally referred to as “pre-emphasis,” The pre-emphasis can be applied for various reasons. For example it can be applied to compensate a frequency characteristic of the transmission line, either ab initia or arising from a particular impairment. The pre-emphasis can be variable, for example to assist in maintaining an adequate eye-opening as data rates are increased. Variable pre-emphasis can be used to synthesize different channel conditions, for example to test with different training sequences.
Concurrently, though, a general objective in the design and operation of a system for communicating high rate signals over a transmission line is a match between the output impedance of the line driver, the impedance of the transmission line, and the input impedance of the receiver(s), in other words the loads on the transmission line. If there is an impedance mismatch there is reduced efficiency in signal power transfer and, often, a related distortion of the received signal.
With respect to the impedance of the transmission line and the impedance at the input of transmission line receivers, various techniques are known for establishing and controlling these values. These techniques can utilize the fact that the transmission line and the input of the receiver(s) can be passive elements. Designs providing acceptable stability of their respective impedance can therefore be straightforward.
However, voltage mode drivers can be preferable for pre-emphasis, and both setting and maintaining a target impedance of voltage mode drivers can have difficulties.